Numerous high-temperature fluid flow processes exist where it is desirable to measure the amount (e.g., concentration or rate of change of concentration versus time) of various species, e.g., gas species. Particular applications include chemical reaction processes, typically gaseous processes, e.g., pyrolysis or cracking of hydrocarbons, e.g., from petroleum or other chemical feedstocks. Other applications include gas flows in combustion process, e.g., external combustion, e.g., furnaces, boilers, burners, and the like; internal combustion engines, e.g., four stroke engines, two stroke engines, diesel engines, and the like, and turbine engines, e.g., jet engines, gas turbines, and the like.
For example, high-temperature capable NOx sensors are required to measure NOx emissions from road vehicles, off-road vehicles (e.g., construction equipment), and power generating equipment. NOx, which can include one or both of NO and NO2, plays an important role in atmospheric reactions that cause harmful particulate matter, ground level ozone and other smog-forming pollutants, and acid rain, and is the focus of legislation in the US and Europe that focuses on large decreases in NOx emission levels. Robust NOx sensors are much in demand in the industry.
Many techniques exist for sensing such gases, for example, mass spectrometry, hot wire detectors, optical spectroscopy, adsorption onto coated microbalances, and others. However, these techniques are not readily applicable for all applications, for example, mass spectrometry can be too complex and fragile for large scale measuring of gases in internal combustion engines, optical spectroscopy can be obscured by particulates produced in combustion processes, and adsorption onto coated microbalances is incompatible with high temperatures and high variations in gas concentration.